In general, a fuel cell is a device that obtains electrical power from fuel, hydrogen and oxygen. Fuel cells are being widely developed as an energy supply device because fuel cells are environmentally superior and can achieve high energy efficiency.
There are examined methods of providing hydrogen to the fuel cell, such as a method of providing hydrogen stored in a storage portion such as a high-pressure hydrogen tank, a hydrogen storing alloy tank or a liquid hydrogen tank. Liquid hydrogen is examined as a hydrogen supply for the fuel cell, because liquid hydrogen has high energy storing density and high hydrogen-charging efficiency to a storage portion.
However, boil-off gas is possibly generated because of vaporization of the liquid hydrogen, when the liquid hydrogen tank is heated from outside. The pressure in the liquid hydrogen tank increases when the boil-off gas is generated. It is, therefore, necessary to exhaust the boil-off gas, when necessary.
Patent Document 1 discloses a method of storing boil-off gas in a pressure container and providing the boil-off gas stored in the pressure container to the fuel cell at startup of the fuel cell. It is possible to use the boil-off gas for a fuel of the fuel cell.    Patent Document 1: Japanese Patent Application Publication No. 2003-56799